The present invention relates to waveform measurement systems and to the field of analog-to-digital converters.
A number of various types of analog-to-digital converters are known to those skilled in the art including integration A/D circuits, parallel A/D circuits, binary ramp A/D circuits and successive approximation A/D circuits.
It is a primary object of the present invention to provide a new method and apparatus for measuring extremely high-speed analog signals with high precision, and converting them into digital or analog form at a slower rate, such signals having rise times of less than two nano-seconds, with an accuracy of 0.01% of full scale.
In waveform digitizing systems the input analog signal is converted into digital form so that the waveform may be processed by digital circuitry. Such waveform acquisition is typically achieved by the use of an analog-to-digital converter sampling instanteneous values of the input signal at predetermined points along the waveform. Acquisition techniques known to those skilled in the art includes real-time acquisition, wherein all the waveform points are acquired in a single sweep, and equivalent time acquisition whereby all of the points are acquired, one point per sweep, during successive sweeps. Such equivalent time acquisition is used for higher sweep rates for high-speed acquisition, rather than real-time acquisition.
Successive approximation A/D converters are also known which employ trial and error to measure the analog signal. In accordance with this technique a successive approximation register makes a trial conversion, tests the results, and then modifies its output according to the results of the test. The trials continue until the available bits are exhausted and the conversion is finished. In other words, in accordance with this technique, the successive approximation register hunts to set the output of a D/A converter to match the input signal. This is accomplished one bit at a time, starting with the most significant bit, each time the approximation register is clocked. Upon completion of this process, a data strobe is outputted indicating that the digital data in the register represents the true value of the analog input signal. This process is slow, limited primarily by the speed of the D/A converter, and the number of bits in the digital word. Additionally the input signal must remain constant over the trial and error period.
An equivalent time sampler requires a signal that repeats, at least for the time interval of interest, after a recurring trigger event. While this technique is relatively fast, the voltage measurement accuracy is limited by the design of a track-and-hold-circuit, requiring the use of non-linear switching elements. When the input signals have extremely high frequencies, the storage capacitor of the track-and-hold circuit is very small since it must charge and discharge extremely rapidly. As a result, the leakage of a small charge quantity produces inaccuracies during the conversion process.
To the inventor's knowledge, the precision measurement (0.01% of full scale) of extremely high frequency signals having rise times of less than two nano-seconds, has not been achieved in the field, prior to his invention. Furthermore the inventor was told by persons skilled in the art that it couldn't be done by presently available test instrumentation.